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carbon nanotubes composite coatings
SCT-21311; No of Pages 5 Surface & Coatings Technology xxx (2016) xxx–xxx
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Microstructure and photocatalytic activities of thermal sprayed titanium dioxide/carbon nanotubes composite coatings P. Daram a, C. Banjongprasert a,b, W. Thongsuwan a,b, S. Jiansirisomboon c,⁎ a b c
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand School of Ceramic Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
a r t i c l e
i n f o
Article history: Received 26 December 2015 Revised 22 June 2016 Accepted in revised form 24 June 2016 Available online xxxx Keywords: Titanium dioxide Carbon nanotubes Plasma spray coating Photocatalytic Nanocomposite
a b s t r a c t This research aims to study the microstructure and photocatalytic performances of titanium dioxide/carbon nanotubes (TiO2/CNTs) composite coating. A TiO2/CNTs composite coating was prepared by plasma spraying from Ni catalytic nanoparticles supported on TiO2 (rutile phase) particles which were prepared by an impregnation method. CNTs were synthesized on the TiO2/Ni powder from an ethanol atmosphere at 650 °C for 60 min by chemical vapor deposition technique. The pure TiO2 and composite powders and their coatings were characterized by SEM, TEM, EDS, XRD, and UV–vis spectroscopy. The result shows that CNTs with a diameter of 50–130 nm were successfully grown in-situ on the surface of TiO2 particles. The coatings mainly consisted of rutile-TiO2, while the Ni catalyst was found in the composite coating. It was found that the thickness and percentage of porosity of the composite coating were similar to those of pure coating. However, the efficiency in methylene blue decomposition of the composite coating was much higher than that of pure TiO2 coating. The result demonstrates a new composite coating with a better photocatalytic performance compared to a pure rutile TiO2 coating. It is expected that this composite coating can be further used for water pollutant applications. © 2016 Published by Elsevier B.V.
1. Introduction Titanium dioxide (TiO2) is an important semiconducting material, which has been applied as photocatalyst and photosensitive materials due to its excellent physical and chemical properties such as moderate band gap (3.2 eV), nontoxic nature, high specific surface area, low cost, high corrosion resistance, high photoactivity, photochemical stability, and ease of handling in different configurations [1]. However, the photocatalytic activity of rutile-TiO2 is not sufficient for industrial purposes due to the rutile TiO2 exhibiting a lower photocatalytic activity compared to anatase TiO2 [2]. Carbon nanotubes (CNTs) are considered a good candidate for a catalyst [3,4] because the heterojunction of TiO2 and CNTs can provide a potential driving force for the separation of photo-induced charge carriers, retarding or hindering the recombination of electrons and holes [5–8]. The CNT–TiO2 Schottky barrier junction is also effective in increasing the recombination time [5–11]. The coupling of TiO2 with carbon nanotubes can provide a synergistic effect, which can enhance the quantum efficiency of a photocatalytic process [6]. The rutile TiO2/CNTs composite is known to have synergistic effects to enhance photocatalytic activity compared to rutile phases.
⁎ Corresponding author. E-mail address: [email protected] (S. Jiansirisomboon).
Based on previous studies, TiO2/CNTs is well known for its high photocatalytic activity [7,8]. There are many techniques for fabricating the TiO2/CNTs coating, e.g. the doctor blade technique [12] and the screening process [13] etc. However, the applications are limited due to the difficulty for producing the composite on a large scale. Thermal spray technology has proved to be a possible process to produce nanocomposite on bulk scale [14–16]. Thus, thermal spray may serve as a commercially viable process to produce TiO2/CNTs composite, but it is yet to be observed. This research therefore attempted to investigate the formation of TiO2/CNT coatings on an industrial production process, plasma spraying. In this work, TiO2/CNT composite feedstock powder was synthesized by a chemical vapor deposition (CVD) method. This work can also provide useful information for fabrication of other new thermal sprayed composite coatings using a similar procedure. 2. Experimental procedure 2.1. Feedstock powder synthesis and characterizations TiO2 particles from Amperite 782 (Germany, Average size 40 mm), 97% Ni(NO3)2·6H2O (Sigma Aldrich, USA) were used for the experiment. The schematic diagram for the synthesis of TiO2/CNTs composites feedstock powder is shown in Fig. 1. First, metal (Ni) nitrate precursors (Ni(NO3)2·6H2O) were added into a TiO2 suspension (10:90 wt% ratios)
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Fig. 1. Schematic diagram for preparation of TiO2/CNTs composite powders by CVD.
and then sonicated for 3 h to obtain a metal ion doped TiO2 suspension via surface absorption. This solution was then dried by heating up to 120 °C. The resultant Ni-incorporated TiO2 powder was then used for the synthesis of TiO2/CNTs composite via chemical vapor deposition (CVD), growing CNTs onto TiO2 particles [17]. Morphologies of as-received TiO2 and TiO2/CNTs composite feedstock powders were characterized using a scanning electron microscope (SEM, JEOL JSM-6335F, Japan). CNTs was characterized using a transmission electron microscope (TEM, JEOL JSM-2010, Japan). Phases of the feedstock powders were determined using energy dispersive spectroscopy (EDS-SEM, JEOL JSM-5910, Japan), X-ray diffractometry (XRD, Philips model X-pert Diffractometer, Netherlands) with CuKα radiation.
2.2. Coating preparation and characterization TiO2 and TiO2/CNTs composite coatings were prepared by plasma spraying. Plasma spray parameters are listed in Table 1. The microstructure and composition of the polished cross-sections of the deposited coatings were studied using a scanning electron microscope (SEM, JEOL JSM-5910LV, Japan) and energy dispersive spectroscopy analysis (EDS-SEM, JEOL JSM-5910, Japan) operated at 15 kV. Phase characterization was also performed using the X-ray diffractometry (XRD, Philips model X-pert Diffractometer, Netherlands) with CuKα radiation. Table 1 Plasma spray parameter. Parameter
Value
Argon flow rate (l min−1) Argon pressure (psi) Hydrogen flow rate (l min−1) Hydrogen pressure (psi) Arc amps (A) Arc volt (V) Powder feed rate (g min−1) Spray distance (cm)
80 100 15 50 500 65–75 40 10
Photocatalytic activity was studied using photocatalytic degradation of methylene blue aqueous solution under UV light irradiation (at 8 W, 280 nm). The initial concentration of methylene blue was about 1 × 10−5 M and this value remained. UV–vis absorbance spectra were obtained for the photocatalytic on degradation of methylene blue (MB) using a UV–vis spectrometer. 3. Results and discussions 3.1. Feedstock powder characterization Fig. 2(a) shows a SEM micrograph of rutile-TiO2 powder. After being synthesized at 650 °C via CVD technique in an ethanol atmosphere, carbon nanotubes (CNTs) were grown on the surface of rutile-TiO2 as shown in Fig. 2(b). TEM studies show that all the samples contained CNTs with a variety of length and diameters. The diameters of CNTs were found in the range of 50–130 nm. The diffraction pattern of an embedded particle is clearly seen corresponding to the FCC-Ni from the Ni catalyst (as shown in Fig. 2(c)). Therefore, the embedded particles were Ni catalyst. Several models were proposed for the role of metal catalysts in the growth of CNTs by CVD method mostly based on the mechanism suggested by Baker et al. [18] and Helveg et al. [19]. Nevertheless, the growth of CNTs in this paper is similar to Srivastava et al. [20], showing the embedded particles in different sections of the tube. The embedded particles were elongated along the tube axis forming nanorods. Moreover, Kang et al. [21] reported that the precipitation of graphite layers has high surface and elastic energy that induced an elongation of the catalyst inside CNTs. It is believed that the catalytic particles are fragmented at different stages of the CNTs growth irregularly. The length of such nanorods was found in the range of 30–200 nm and the diameter is equal to the inner diameter of the CNTs as shown in Fig. 2(c). 3.2. Coating characterizations The microstructure of plasma sprayed TiO2/CNTs composite coating is shown in Fig. 3 The composite coating has a low porosity (2 vol%) with uniform and dense microstructure as shown in Fig. 3(b). The
Fig. 2. SEM micrographs of (a) rutile-TiO2 powder and (b) TiO2/CNTs composite powder and (c) TEM micrographs of CNTs and SADP of Ni catalyst.
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Fig. 3. Cross-sectional SEM micrographs of TiO2/CNTs composite coating at (a) 100× magnification and (b) 1500× magnification and (c) SEM micrograph of fracture surface in TiO2/CNTs composite coating.
grey and white regions were analyzed using EDS in the SEM. This analysis revealed large Ti peaks and O peaks (shown in Fig. 4(a)) on the grey regions indicating TiO2. The white regions rendered large Ni peaks in the EDS spectrum indicating Ni. The results of phase analysis of coatings by the X-ray diffraction method are shown in Fig. 5 indicating the same phase, i.e. rutile-TiO2 and Magneli phase, for both coatings. Additionally, both coatings show the presence of rutile-TiO2 and Magneli phase [10, 22] as observed in feedstock powders before spraying and the FCC-Ni in the composite coating according to TEM and EDS result. However, CNTs were observed in the fractured surface of the composite coating as shown in Fig. 3(c) confirming the existence of CNTs in the coating after spraying similar to the report for CNTs reinforced composite coatings prepared by other techniques [23–26]. It seemed that some amount of CNTs were accumulated in certain zones near rough edge areas between splats as well as lightly distributed between splats. During particle flight, nearly all TiO2 particles became liquid and there were factors determining distribution of CNTs including gravity, surface tension, and air-dragged force. The last factor might be the most important because during molten particle flight, CNTs in front of the liquid would be pushed to the side of the liquid and upon impact on the substrate,
most CNTs could be agglomerated between splats along the side and fewer CNTs would exist in the front (also between splats) and inside the splat [25]. This explanation agreed well with what was observed (Fig. 2(c)). With the reduction of CNTs in this coating, some CNTs could be partially damaged during spraying due to the high deposition temperature of the plasma ~ 5000–25,000 °C [27]. During plasma spray, in-flight TiO2/CNT composite powder was subjected to heat only for a very short period of time (~3.56 × 10−4 s [28]) considering the short distance used in this experiment, partial loss of CNTs due to combustion that could occur during spraying [26]. Nevertheless, most of CNTs remained and this is similar to the reports from [28,29] in which CNTs become entrapped between the splat interfaces. 3.3. Photocatalytic activity The influence of CNTs in the composite photocatalytic activity was studied using photocatalytic degradation of methylene blue aqueous solution under UV light irradiation. Fig. 6 shows the results of the photocatalytic decay of methylene blue under UV irradiation in the presence of TiO2 coating and TiO2/CNTs composite coating. TiO2/CNTs composite
Fig. 4. Cross-sectional SEM micrographs and EDS spectra of TiO2/CNTs composite coatings at (a) point analysis of grey regions, (b) point analysis of white regions.
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because the coatings were high in porosity resulting in a low wear and corrosion resistance. Moreover, it is very difficult to maintain a metastable anatase phase in the final coating. Consequently, CNTs is a strong candidate to be used to enhance the photocatalytic activity of the rutile-TiO2 coating especially by plasma spraying that is a standard process for industrial applications. 4. Conclusions The results show that CNTs can be grown in-situ on the surface of rutile-TiO2 powder. This powder was plasma sprayed to form a new TiO2/ CNTs composite coating. Phase composition of the coating comprised mainly rutile-TiO2 with a Ni catalyst compared to that observed in the TiO2 coating. The remaining of CNTs in the composite coating was clearly observed in SEM image. Although percentages of porosity of both coating were similar, the TiO2/CNTs coating exhibited higher photocatalytic activities than the rutile-TiO2 coating, where the concentration of MB solution was decreased about 35%. However, the composite coating exhibited lower activity than the composite and pure-TiO2 powder. Fig. 5. XRD pattern of powders and coatings.
Acknowledgements coating exhibited higher activity than that of TiO2 coatings. From other works [5–9], it was indicated that the photocatalytic activity of CNTs/ TiO2 composite powder was much better than that of TiO2 powder. The CNTs could act as electron acceptors, promoting the separation of photoinduced electron-hole pair and retarding their recombination [6, 7]. Moreover, CNTs can act as a dispersing agent to prevent TiO2 from agglomerating, thus providing a highly active surface area on TiO2/ CNTs composite [8–11]. Nevertheless, TiO2/CNTs composite coatings exhibited lower activity than that of both powders, the photocatalytic activity of the composite coating is the lowest among the two powders, which may be attributed to the two following reasons: (i) splat of the coatings was larger than the powders leading to a lower surface area to react with light, (ii) lower amount of CNTs in the coatings after plasma spraying. The photocatalytic activity was observed; there was lower activity than that reported on nano-structured pure TiO2 coatings by plasma spray [30] and HVOF thermal spray [31]. This shows that the coating has higher photocatalytic activity due to a higher anatase phase fraction and small grain size, which were considered to have an influence on the photocatalytic reactions, in the as-sprayed coatings according to the process. However, this may cause a severe wear and corrosion in service
Fig. 6. Comparison of photocatalytic decay of methylene blue for rutile-TiO2 powder, TiO2/ CNTs composite powders, TiO2 coating and TiO2/CNTs composite coating under UV light irradiation at 120 min.
The author would like to thank the Faculty of Science and the Graduated School, Chiang Mai University and the Metal and Materials Technology Center (MTEC), and National Science and Technology Development Agency (NSTDA). National Research University Project under Thailand's Office of the High Education Commission and Thailand Research Fund (IRG5780013) are also acknowledged for financial supports. References [1] T. Kasuga, M. Hiramatsu, M. Hirano, A. Hoson, Preparation of TiO2-based powders with high photocatalytic activities, J. Mater. Res. 12 (1997) 607–609. [2] L. Liu, H. Zhao, J.M. Andino, Y. Li, Photocatalytic CO2 reduction with H2O on TiO2 nanocrystals: comparison of anatase, rutile, and brookite polymorphs and exploration of surface chemistry, ACS Catal. 2 (2012) 1817–1828. [3] M. Kumar, Chemical vapor deposition of carbon nanotubes, a review on growth mechanism and mass production, J. Nanosci. Nanotechnol. 10 (2010) 3739–3758. [4] S.W. Pattinson, Mechanism and enhanced yield of carbon nanotubes growth, Chem. Mater. 27 (2015) 932–937. [5] K. Woan, G. Pyrgiotakis, W. Sigmun, Photocatalytic carbon nanotube–TiO2 composites, Adv. Mater. 21 (2009) 2233–2239. [6] W.D. Wang, P. Serp, P. Kalck, J.L. Faria, Visible light photodegradation of phenol on MWCNT–TiO2 composite catalysts prepared by a modified sol–gel method, J. Mol. Catal. A Chem. 235 (2005) 9–194. [7] L.C. Jiang, W.D. Zhang, Charge transfer properties and photoelectrocatalytic activity of TiO2/MWCNT hybrid, Electrochim. Acta 56 (2010) 406–411. [8] N. Bouazza, M. Ouzzine, M.A. Lillo-Rodenas, D. Eder, A. Linares-Solano, TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration, Appl. Catal. B Environ. 92 (2009) 377–383. [9] R. Leary, A. Westwood, Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis, Carbon 49 (2011) 741–772. [10] Y. Yu, J.C. Yu, C.Y. Chan, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye, Appl. Catal. B Environ. 61 (2005) 1–11. [11] Y. Yu, J.C. Yu, C.Y. Chan, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal. A Gen. 289 (2005) 186–196. [12] M.J. Sampaio, C.G. Silva, R.R.N. Marques, A.M.T. Silva, J.L. Faria, Carbon nanotube– TiO2 thin films for photocatalytic applications, Catal. Today 161 (2011) 91–96. [13] Y.W. Kim, S.H. Park, The development of photocatalyst with hybrid material CNT/ TiO2 thin films for dye-sensitized solar cell, J. Nanomater. 24 (2013) 1–5. [14] J.A. Gan, C.C. Berndt, Nanocomposite coatings: thermal spray processing: microstructure and performance, Int. Mater. Rev. 60 (2015) 195–244. [15] C. Banjongprasert, P. Jaimeewong, S. Jiansirisomboon, Investigation of thermal sprayed stainless steel/WC-12 wt%Co nanocomposite coatings, Mater. Sci. Forum 695 (2011) 441–444. [16] A. Limpichaipanit, C. Banjongprasert, P. Jaiban, S. Jiansirisomboon, Fabrication and properties of thermal sprayed AlSi-based coatings from nanocomposite powders, J. Therm. Spray Technol. 22 (2013) 18–26. [17] P. Daram, W. Thongsuwan, S. Jiansirisomboon, Influence of carbon nanotubes on photocatalytic activities of titanium dioxide nanocomposite powders, Key Eng. Mater. 659 (2015) 315–320. [18] R.T.K. Baker, P.S. Harris, in: J.P.L. Walker, P.A. Thrower (Eds.), Chemistry and Physics of Carbon, Dekker, New York 1978, p. 83.
Please cite this article as: P. Daram, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.068
P. Daram et al. / Surface & Coatings Technology xxx (2016) xxx–xxx [19] S. Helveg, C.L.´. pez-Cartes, J. Sehested, P.L. Hansen, B.S. Clausen, J.R. Rostrup-Nielsen, F. Abild-Pedersen, J.K. Nørskov, Atomic-scale imaging of carbon nanofibre growth, Nature 427 (2004) 426–429. [20] S.K. Srivastava, V.D. Vankar, V. Kumar, Effect of hydrogen plasma treatment on the growth and microstructures of multiwalled carbon nanotubes, Nano-Micro Lett. 2 (2010) 42–48. [21] J.L. Kang, J.J. Li, X.W. Du, C.S. Shi, N.Q. Zhao, L. Cui, P. Nash, Synthesis and growth mechanism of metal filled carbon nanostructures by CVD using Ni/Y catalyst supported on copper, J. Alloys Compd. 456 (2008) 290–296. [22] G. Bertrand, N. Berger-Keller, C. Meunier, C. Coddet, Evaluation of metastable phase and microhardness on plasma sprayed titania coatings, Surf. Coat. Technol. 200 (2006) 5013–5019. [23] D. Kaewsai, A. Watcharapasorn, P. Singjai, S. Wirojanupatump, P. Niranatlumpong, S. Jiansirisomboon, Thermal sprayed stainless steel/carbon nanotube composite coatings, Surf. Coat. Technol. 205 (2010) 2104–2112. [24] D. Deesom, S. Moonngam, K. Charoenrut, C. Banjongprasert, Fabrication and properties of NiCr/CNTs nanocomposite coatings prepared by high velocity oxy-fuel spraying, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06. 016.
5
[25] T. Laha, A. Agarwal, T. McKechnie, S. Seal, Synthesis and characterization of plasma spray formed carbon nanotube reinforced aluminum composite, Mater. Sci. Eng. A 381 (2004) 249–258. [26] A.K. Keshri, K. Balani, S.R. Bakshi, V. Singh, T. Laha, S. Seal, A. Agarwal, Structural transformations in carbon nanotubes during thermal spray processing, Surf. Coat. Technol. 203 (2009) 2193–2201. [27] J.M. Wu, H.C. Shih, W.T. Wu, Y.K. Tseng, I.C. Chen, Thermal evaporation growth and the luminescence property of TiO2 nanowires, J. Cryst. Growth 281 (2005) 384–390. [28] K. Balani, T. Zhang, A. Karakoti, W.Z. Li, S. Seal, A. Agarw, In situ carbon nanotube reinforcements in a plasma-sprayed aluminum oxide nanocomposite coating, Acta Mater. 56 (2008) 571–579. [29] K. Balani, A. Agarwal, Wetting of carbon nanotubes by aluminum oxide, Nanotechnology 19 (2008) 165701. [30] C. Lee, H. Choi, C. Lee, H. Kim, Photocatalytic properties of nano-structured TiO2 plasma sprayed coating, Surf. Coat. Technol. 173 (2003) 192–200. [31] M. Bozorgtabar, M. Rahimipour, M. Salehi, Novel photocatalytic TiO2 coatings produced by HVOF thermal spraying process, Mater. Lett. 64 (2010) 1173–1175.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and photocatalytic activities of thermal sprayed titanium dioxide/carbon nanotubes composite coatings P. Daram a, C. Banjongprasert a,b, W. Thongsuwan a,b, S. Jiansirisomboon c,⁎ a b c
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand School of Ceramic Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand
a r t i c l e
i n f o
Article history: Received 26 December 2015 Revised 22 June 2016 Accepted in revised form 24 June 2016 Available online xxxx Keywords: Titanium dioxide Carbon nanotubes Plasma spray coating Photocatalytic Nanocomposite
a b s t r a c t This research aims to study the microstructure and photocatalytic performances of titanium dioxide/carbon nanotubes (TiO2/CNTs) composite coating. A TiO2/CNTs composite coating was prepared by plasma spraying from Ni catalytic nanoparticles supported on TiO2 (rutile phase) particles which were prepared by an impregnation method. CNTs were synthesized on the TiO2/Ni powder from an ethanol atmosphere at 650 °C for 60 min by chemical vapor deposition technique. The pure TiO2 and composite powders and their coatings were characterized by SEM, TEM, EDS, XRD, and UV–vis spectroscopy. The result shows that CNTs with a diameter of 50–130 nm were successfully grown in-situ on the surface of TiO2 particles. The coatings mainly consisted of rutile-TiO2, while the Ni catalyst was found in the composite coating. It was found that the thickness and percentage of porosity of the composite coating were similar to those of pure coating. However, the efficiency in methylene blue decomposition of the composite coating was much higher than that of pure TiO2 coating. The result demonstrates a new composite coating with a better photocatalytic performance compared to a pure rutile TiO2 coating. It is expected that this composite coating can be further used for water pollutant applications. © 2016 Published by Elsevier B.V.
1. Introduction Titanium dioxide (TiO2) is an important semiconducting material, which has been applied as photocatalyst and photosensitive materials due to its excellent physical and chemical properties such as moderate band gap (3.2 eV), nontoxic nature, high specific surface area, low cost, high corrosion resistance, high photoactivity, photochemical stability, and ease of handling in different configurations [1]. However, the photocatalytic activity of rutile-TiO2 is not sufficient for industrial purposes due to the rutile TiO2 exhibiting a lower photocatalytic activity compared to anatase TiO2 [2]. Carbon nanotubes (CNTs) are considered a good candidate for a catalyst [3,4] because the heterojunction of TiO2 and CNTs can provide a potential driving force for the separation of photo-induced charge carriers, retarding or hindering the recombination of electrons and holes [5–8]. The CNT–TiO2 Schottky barrier junction is also effective in increasing the recombination time [5–11]. The coupling of TiO2 with carbon nanotubes can provide a synergistic effect, which can enhance the quantum efficiency of a photocatalytic process [6]. The rutile TiO2/CNTs composite is known to have synergistic effects to enhance photocatalytic activity compared to rutile phases.
⁎ Corresponding author. E-mail address: [email protected] (S. Jiansirisomboon).
Based on previous studies, TiO2/CNTs is well known for its high photocatalytic activity [7,8]. There are many techniques for fabricating the TiO2/CNTs coating, e.g. the doctor blade technique [12] and the screening process [13] etc. However, the applications are limited due to the difficulty for producing the composite on a large scale. Thermal spray technology has proved to be a possible process to produce nanocomposite on bulk scale [14–16]. Thus, thermal spray may serve as a commercially viable process to produce TiO2/CNTs composite, but it is yet to be observed. This research therefore attempted to investigate the formation of TiO2/CNT coatings on an industrial production process, plasma spraying. In this work, TiO2/CNT composite feedstock powder was synthesized by a chemical vapor deposition (CVD) method. This work can also provide useful information for fabrication of other new thermal sprayed composite coatings using a similar procedure. 2. Experimental procedure 2.1. Feedstock powder synthesis and characterizations TiO2 particles from Amperite 782 (Germany, Average size 40 mm), 97% Ni(NO3)2·6H2O (Sigma Aldrich, USA) were used for the experiment. The schematic diagram for the synthesis of TiO2/CNTs composites feedstock powder is shown in Fig. 1. First, metal (Ni) nitrate precursors (Ni(NO3)2·6H2O) were added into a TiO2 suspension (10:90 wt% ratios)
http://dx.doi.org/10.1016/j.surfcoat.2016.06.068 0257-8972/© 2016 Published by Elsevier B.V.
Please cite this article as: P. Daram, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.068
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Fig. 1. Schematic diagram for preparation of TiO2/CNTs composite powders by CVD.
and then sonicated for 3 h to obtain a metal ion doped TiO2 suspension via surface absorption. This solution was then dried by heating up to 120 °C. The resultant Ni-incorporated TiO2 powder was then used for the synthesis of TiO2/CNTs composite via chemical vapor deposition (CVD), growing CNTs onto TiO2 particles [17]. Morphologies of as-received TiO2 and TiO2/CNTs composite feedstock powders were characterized using a scanning electron microscope (SEM, JEOL JSM-6335F, Japan). CNTs was characterized using a transmission electron microscope (TEM, JEOL JSM-2010, Japan). Phases of the feedstock powders were determined using energy dispersive spectroscopy (EDS-SEM, JEOL JSM-5910, Japan), X-ray diffractometry (XRD, Philips model X-pert Diffractometer, Netherlands) with CuKα radiation.
2.2. Coating preparation and characterization TiO2 and TiO2/CNTs composite coatings were prepared by plasma spraying. Plasma spray parameters are listed in Table 1. The microstructure and composition of the polished cross-sections of the deposited coatings were studied using a scanning electron microscope (SEM, JEOL JSM-5910LV, Japan) and energy dispersive spectroscopy analysis (EDS-SEM, JEOL JSM-5910, Japan) operated at 15 kV. Phase characterization was also performed using the X-ray diffractometry (XRD, Philips model X-pert Diffractometer, Netherlands) with CuKα radiation. Table 1 Plasma spray parameter. Parameter
Value
Argon flow rate (l min−1) Argon pressure (psi) Hydrogen flow rate (l min−1) Hydrogen pressure (psi) Arc amps (A) Arc volt (V) Powder feed rate (g min−1) Spray distance (cm)
80 100 15 50 500 65–75 40 10
Photocatalytic activity was studied using photocatalytic degradation of methylene blue aqueous solution under UV light irradiation (at 8 W, 280 nm). The initial concentration of methylene blue was about 1 × 10−5 M and this value remained. UV–vis absorbance spectra were obtained for the photocatalytic on degradation of methylene blue (MB) using a UV–vis spectrometer. 3. Results and discussions 3.1. Feedstock powder characterization Fig. 2(a) shows a SEM micrograph of rutile-TiO2 powder. After being synthesized at 650 °C via CVD technique in an ethanol atmosphere, carbon nanotubes (CNTs) were grown on the surface of rutile-TiO2 as shown in Fig. 2(b). TEM studies show that all the samples contained CNTs with a variety of length and diameters. The diameters of CNTs were found in the range of 50–130 nm. The diffraction pattern of an embedded particle is clearly seen corresponding to the FCC-Ni from the Ni catalyst (as shown in Fig. 2(c)). Therefore, the embedded particles were Ni catalyst. Several models were proposed for the role of metal catalysts in the growth of CNTs by CVD method mostly based on the mechanism suggested by Baker et al. [18] and Helveg et al. [19]. Nevertheless, the growth of CNTs in this paper is similar to Srivastava et al. [20], showing the embedded particles in different sections of the tube. The embedded particles were elongated along the tube axis forming nanorods. Moreover, Kang et al. [21] reported that the precipitation of graphite layers has high surface and elastic energy that induced an elongation of the catalyst inside CNTs. It is believed that the catalytic particles are fragmented at different stages of the CNTs growth irregularly. The length of such nanorods was found in the range of 30–200 nm and the diameter is equal to the inner diameter of the CNTs as shown in Fig. 2(c). 3.2. Coating characterizations The microstructure of plasma sprayed TiO2/CNTs composite coating is shown in Fig. 3 The composite coating has a low porosity (2 vol%) with uniform and dense microstructure as shown in Fig. 3(b). The
Fig. 2. SEM micrographs of (a) rutile-TiO2 powder and (b) TiO2/CNTs composite powder and (c) TEM micrographs of CNTs and SADP of Ni catalyst.
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Fig. 3. Cross-sectional SEM micrographs of TiO2/CNTs composite coating at (a) 100× magnification and (b) 1500× magnification and (c) SEM micrograph of fracture surface in TiO2/CNTs composite coating.
grey and white regions were analyzed using EDS in the SEM. This analysis revealed large Ti peaks and O peaks (shown in Fig. 4(a)) on the grey regions indicating TiO2. The white regions rendered large Ni peaks in the EDS spectrum indicating Ni. The results of phase analysis of coatings by the X-ray diffraction method are shown in Fig. 5 indicating the same phase, i.e. rutile-TiO2 and Magneli phase, for both coatings. Additionally, both coatings show the presence of rutile-TiO2 and Magneli phase [10, 22] as observed in feedstock powders before spraying and the FCC-Ni in the composite coating according to TEM and EDS result. However, CNTs were observed in the fractured surface of the composite coating as shown in Fig. 3(c) confirming the existence of CNTs in the coating after spraying similar to the report for CNTs reinforced composite coatings prepared by other techniques [23–26]. It seemed that some amount of CNTs were accumulated in certain zones near rough edge areas between splats as well as lightly distributed between splats. During particle flight, nearly all TiO2 particles became liquid and there were factors determining distribution of CNTs including gravity, surface tension, and air-dragged force. The last factor might be the most important because during molten particle flight, CNTs in front of the liquid would be pushed to the side of the liquid and upon impact on the substrate,
most CNTs could be agglomerated between splats along the side and fewer CNTs would exist in the front (also between splats) and inside the splat [25]. This explanation agreed well with what was observed (Fig. 2(c)). With the reduction of CNTs in this coating, some CNTs could be partially damaged during spraying due to the high deposition temperature of the plasma ~ 5000–25,000 °C [27]. During plasma spray, in-flight TiO2/CNT composite powder was subjected to heat only for a very short period of time (~3.56 × 10−4 s [28]) considering the short distance used in this experiment, partial loss of CNTs due to combustion that could occur during spraying [26]. Nevertheless, most of CNTs remained and this is similar to the reports from [28,29] in which CNTs become entrapped between the splat interfaces. 3.3. Photocatalytic activity The influence of CNTs in the composite photocatalytic activity was studied using photocatalytic degradation of methylene blue aqueous solution under UV light irradiation. Fig. 6 shows the results of the photocatalytic decay of methylene blue under UV irradiation in the presence of TiO2 coating and TiO2/CNTs composite coating. TiO2/CNTs composite
Fig. 4. Cross-sectional SEM micrographs and EDS spectra of TiO2/CNTs composite coatings at (a) point analysis of grey regions, (b) point analysis of white regions.
Please cite this article as: P. Daram, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.068
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because the coatings were high in porosity resulting in a low wear and corrosion resistance. Moreover, it is very difficult to maintain a metastable anatase phase in the final coating. Consequently, CNTs is a strong candidate to be used to enhance the photocatalytic activity of the rutile-TiO2 coating especially by plasma spraying that is a standard process for industrial applications. 4. Conclusions The results show that CNTs can be grown in-situ on the surface of rutile-TiO2 powder. This powder was plasma sprayed to form a new TiO2/ CNTs composite coating. Phase composition of the coating comprised mainly rutile-TiO2 with a Ni catalyst compared to that observed in the TiO2 coating. The remaining of CNTs in the composite coating was clearly observed in SEM image. Although percentages of porosity of both coating were similar, the TiO2/CNTs coating exhibited higher photocatalytic activities than the rutile-TiO2 coating, where the concentration of MB solution was decreased about 35%. However, the composite coating exhibited lower activity than the composite and pure-TiO2 powder. Fig. 5. XRD pattern of powders and coatings.
Acknowledgements coating exhibited higher activity than that of TiO2 coatings. From other works [5–9], it was indicated that the photocatalytic activity of CNTs/ TiO2 composite powder was much better than that of TiO2 powder. The CNTs could act as electron acceptors, promoting the separation of photoinduced electron-hole pair and retarding their recombination [6, 7]. Moreover, CNTs can act as a dispersing agent to prevent TiO2 from agglomerating, thus providing a highly active surface area on TiO2/ CNTs composite [8–11]. Nevertheless, TiO2/CNTs composite coatings exhibited lower activity than that of both powders, the photocatalytic activity of the composite coating is the lowest among the two powders, which may be attributed to the two following reasons: (i) splat of the coatings was larger than the powders leading to a lower surface area to react with light, (ii) lower amount of CNTs in the coatings after plasma spraying. The photocatalytic activity was observed; there was lower activity than that reported on nano-structured pure TiO2 coatings by plasma spray [30] and HVOF thermal spray [31]. This shows that the coating has higher photocatalytic activity due to a higher anatase phase fraction and small grain size, which were considered to have an influence on the photocatalytic reactions, in the as-sprayed coatings according to the process. However, this may cause a severe wear and corrosion in service
Fig. 6. Comparison of photocatalytic decay of methylene blue for rutile-TiO2 powder, TiO2/ CNTs composite powders, TiO2 coating and TiO2/CNTs composite coating under UV light irradiation at 120 min.
The author would like to thank the Faculty of Science and the Graduated School, Chiang Mai University and the Metal and Materials Technology Center (MTEC), and National Science and Technology Development Agency (NSTDA). National Research University Project under Thailand's Office of the High Education Commission and Thailand Research Fund (IRG5780013) are also acknowledged for financial supports. References [1] T. Kasuga, M. Hiramatsu, M. Hirano, A. Hoson, Preparation of TiO2-based powders with high photocatalytic activities, J. Mater. Res. 12 (1997) 607–609. [2] L. Liu, H. Zhao, J.M. Andino, Y. Li, Photocatalytic CO2 reduction with H2O on TiO2 nanocrystals: comparison of anatase, rutile, and brookite polymorphs and exploration of surface chemistry, ACS Catal. 2 (2012) 1817–1828. [3] M. Kumar, Chemical vapor deposition of carbon nanotubes, a review on growth mechanism and mass production, J. Nanosci. Nanotechnol. 10 (2010) 3739–3758. [4] S.W. Pattinson, Mechanism and enhanced yield of carbon nanotubes growth, Chem. Mater. 27 (2015) 932–937. [5] K. Woan, G. Pyrgiotakis, W. Sigmun, Photocatalytic carbon nanotube–TiO2 composites, Adv. Mater. 21 (2009) 2233–2239. [6] W.D. Wang, P. Serp, P. Kalck, J.L. Faria, Visible light photodegradation of phenol on MWCNT–TiO2 composite catalysts prepared by a modified sol–gel method, J. Mol. Catal. A Chem. 235 (2005) 9–194. [7] L.C. Jiang, W.D. Zhang, Charge transfer properties and photoelectrocatalytic activity of TiO2/MWCNT hybrid, Electrochim. Acta 56 (2010) 406–411. [8] N. Bouazza, M. Ouzzine, M.A. Lillo-Rodenas, D. Eder, A. Linares-Solano, TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration, Appl. Catal. B Environ. 92 (2009) 377–383. [9] R. Leary, A. Westwood, Carbonaceous nanomaterials for the enhancement of TiO2 photocatalysis, Carbon 49 (2011) 741–772. [10] Y. Yu, J.C. Yu, C.Y. Chan, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye, Appl. Catal. B Environ. 61 (2005) 1–11. [11] Y. Yu, J.C. Yu, C.Y. Chan, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes, Appl. Catal. A Gen. 289 (2005) 186–196. [12] M.J. Sampaio, C.G. Silva, R.R.N. Marques, A.M.T. Silva, J.L. Faria, Carbon nanotube– TiO2 thin films for photocatalytic applications, Catal. Today 161 (2011) 91–96. [13] Y.W. Kim, S.H. Park, The development of photocatalyst with hybrid material CNT/ TiO2 thin films for dye-sensitized solar cell, J. Nanomater. 24 (2013) 1–5. [14] J.A. Gan, C.C. Berndt, Nanocomposite coatings: thermal spray processing: microstructure and performance, Int. Mater. Rev. 60 (2015) 195–244. [15] C. Banjongprasert, P. Jaimeewong, S. Jiansirisomboon, Investigation of thermal sprayed stainless steel/WC-12 wt%Co nanocomposite coatings, Mater. Sci. Forum 695 (2011) 441–444. [16] A. Limpichaipanit, C. Banjongprasert, P. Jaiban, S. Jiansirisomboon, Fabrication and properties of thermal sprayed AlSi-based coatings from nanocomposite powders, J. Therm. Spray Technol. 22 (2013) 18–26. [17] P. Daram, W. Thongsuwan, S. Jiansirisomboon, Influence of carbon nanotubes on photocatalytic activities of titanium dioxide nanocomposite powders, Key Eng. Mater. 659 (2015) 315–320. [18] R.T.K. Baker, P.S. Harris, in: J.P.L. Walker, P.A. Thrower (Eds.), Chemistry and Physics of Carbon, Dekker, New York 1978, p. 83.
Please cite this article as: P. Daram, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.068
P. Daram et al. / Surface & Coatings Technology xxx (2016) xxx–xxx [19] S. Helveg, C.L.´. pez-Cartes, J. Sehested, P.L. Hansen, B.S. Clausen, J.R. Rostrup-Nielsen, F. Abild-Pedersen, J.K. Nørskov, Atomic-scale imaging of carbon nanofibre growth, Nature 427 (2004) 426–429. [20] S.K. Srivastava, V.D. Vankar, V. Kumar, Effect of hydrogen plasma treatment on the growth and microstructures of multiwalled carbon nanotubes, Nano-Micro Lett. 2 (2010) 42–48. [21] J.L. Kang, J.J. Li, X.W. Du, C.S. Shi, N.Q. Zhao, L. Cui, P. Nash, Synthesis and growth mechanism of metal filled carbon nanostructures by CVD using Ni/Y catalyst supported on copper, J. Alloys Compd. 456 (2008) 290–296. [22] G. Bertrand, N. Berger-Keller, C. Meunier, C. Coddet, Evaluation of metastable phase and microhardness on plasma sprayed titania coatings, Surf. Coat. Technol. 200 (2006) 5013–5019. [23] D. Kaewsai, A. Watcharapasorn, P. Singjai, S. Wirojanupatump, P. Niranatlumpong, S. Jiansirisomboon, Thermal sprayed stainless steel/carbon nanotube composite coatings, Surf. Coat. Technol. 205 (2010) 2104–2112. [24] D. Deesom, S. Moonngam, K. Charoenrut, C. Banjongprasert, Fabrication and properties of NiCr/CNTs nanocomposite coatings prepared by high velocity oxy-fuel spraying, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06. 016.
5
[25] T. Laha, A. Agarwal, T. McKechnie, S. Seal, Synthesis and characterization of plasma spray formed carbon nanotube reinforced aluminum composite, Mater. Sci. Eng. A 381 (2004) 249–258. [26] A.K. Keshri, K. Balani, S.R. Bakshi, V. Singh, T. Laha, S. Seal, A. Agarwal, Structural transformations in carbon nanotubes during thermal spray processing, Surf. Coat. Technol. 203 (2009) 2193–2201. [27] J.M. Wu, H.C. Shih, W.T. Wu, Y.K. Tseng, I.C. Chen, Thermal evaporation growth and the luminescence property of TiO2 nanowires, J. Cryst. Growth 281 (2005) 384–390. [28] K. Balani, T. Zhang, A. Karakoti, W.Z. Li, S. Seal, A. Agarw, In situ carbon nanotube reinforcements in a plasma-sprayed aluminum oxide nanocomposite coating, Acta Mater. 56 (2008) 571–579. [29] K. Balani, A. Agarwal, Wetting of carbon nanotubes by aluminum oxide, Nanotechnology 19 (2008) 165701. [30] C. Lee, H. Choi, C. Lee, H. Kim, Photocatalytic properties of nano-structured TiO2 plasma sprayed coating, Surf. Coat. Technol. 173 (2003) 192–200. [31] M. Bozorgtabar, M. Rahimipour, M. Salehi, Novel photocatalytic TiO2 coatings produced by HVOF thermal spraying process, Mater. Lett. 64 (2010) 1173–1175.
Please cite this article as: P. Daram, et al., Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.06.068